The invention relates to devices for detecting the presence of smoke, dust or similar particles in the ambient atmosphere, which devices include an ionization chamber whose inner volume is placed in communication with said atmosphere and means for measuring the ionic current created in this chamber. Said ionization chamber itself includes a source of ionizing radiation, generally formed by an .alpha. or .beta. radioactive substance, capable of ionizing the air contained in the chamber, two electrodes enclosing a portion at least of the volume thus ionized and means for applying a DC electric voltage between these two electrodes, which creates the ionic current to be measured between said electrodes.
The presence of smoke or other polluting agents in the chamber considered results in constituent particles of this agent being fixed on the ions present in this chamber, which makes them heavier, slows them down and reduces the intensity of the corresponding ionic current.
The variation of this intensity is used for detecting such a presence and for triggering an alarm as soon as the concentration of the polluting agent exceeds a predetermined threshold.
Such a device serves preferably for detecting fires from the smoke or fumes which they release and, in order to simplify the description, but of course in a non limitative way, the polluting agent will be designated hereafter by the word "smoke".
The movements of the ambient air in the immediate vicinity of the detector device of the kind in question may adversely affect the detections which it ensures.
In fact, an air stream sweeping the inner volume of the ionization chamber carries along with it some of the ions which pass between the two electrodes of this chamber, which reduces the ionic current even in the absence of smoke: such removal of ions may ultimately trigger off false alarms.
To get over such a drawback, a number of solutions have already been proposed, of a mechanical kind (such as the addition of baffles or deflecting flaps for reducing the violence of the winds admitted into the chamber) and/or of an electric kind (such as the construction of electrodes having a form such that the particles and ions carried along by the air currents are sent to regions where a high electric field reigns).
But these solutions all have drawbacks such as reduction of the sensitivity of the detector and/or increase in the price and fragility.